It is well known to the art that condensation of a phenol and an aldehyde provides materials curable to thermoset phenolic resins. Base-catalyzed condensation employing at least about a stoichiometric amount of aldehyde provides condensates known as resoles whereas acid catalysts and a deficiency of aldehyde provides novolacs. Characteristic of both liquid and solid resoles is their heat-curability to fully cross-linked, infusible products without the need for an added cross-linking agent. From this standpoint, resoles are more descriptively referred to as One-Step phenolic resins in contrast to novolacs or Two-Step resins which do require the addition of a cross-linking agent for the curing process. The reactivity of resoles and ability to self-condense to higher molecular weight resins is attributable to the presence of hydroxymethyl groups which become bonded to the aromatic phenolic nucleus during the base-catalyzed condensation.
Curing of resoles to higher molecular weight, cross-linked thermoset resins proceeds with generation of heat and is accelerated by acid materials. In the presence of strongly acidic accelerators of the exothermic reaction and a source of blowing action, liquid resoles cure rapidly to cellular phenolic resins. Although phenolic foams are used to embed floral arrangements and for general packaging purposes, they have not found the widespread industrial application enjoyed by cellular polyurethanes. As compared with cellular polyurethanes, phenolic foams possess better inherent resistance to burn with an open flame, and emit very low levels of smoke on heating. Consequently, greater attention is being focused on phenolic foam technology, so as to develop practical products having more widespread end-use applications. From the standpoint of commercial application, the most significant resoles are those derived from phenol itself and formaldehyde. In addition to improving certain properties of phenolic foams such as their friability and punking behavior (that is, glowing combustion without a visible flame), there is also a need to provide improvement in the processability of the resole raw materials.
The major component of phenolic foam formulations is the resole itself. Consequently, foaming of the formulation is hampered in those instances where the resole is highly viscous. Viscosity increase to relatively high levels is often observed during storage of the resole raw material. The build-up in viscosity associated with conventional resole resins of poor shelf-life is brought about by the tendency of such materials to advance to an irreversibly higher molecular weight form upon aging. Such self-polymerization is evidenced by a consequential loss in the reactivity of the resole as a foamable composition which in turn is reflected by a corresponding substantial increase in the density of phenolic foam derived therefrom. In those instances where the aged resole may have retained some measure of reactivity as a foamable composition, it may not be properly processable because its viscosity is too high.
The prior art recognizes the problems associated with the poor shelf life of conventional resoles and that aging or self-condensation during storage is associated with substantial increase in resole viscosity. For example, in accordance with U.S. Pat. No. 3,313,766, the stability of phenolic resoles intended for use as binders is said to be improved and viscosity reduced by the addition to the resole of trioxane.
Build-up of viscosity during aging is also associated with the particular ion exchanged resoles described in my prior and copending application Ser. No. 595,744, filed July 14, 1975. Among other distinguishing features and improved properties, the resoles described in said application are noteworthy for their inherently greater stability and excellent shelf-life, as well as for their enhanced reactivity as foamable compositions. The substantial increase in viscosity which is often observed during aging of these improved resoles is not accompanied by a corresponding loss in their reactivity as foamable compositions. This characteristic indicates that viscosity build-up of such improved foamable compositions is brought about by a physical phenomenon such as, for example, intermolecular bonding, as opposed to self-condensation by chemical reaction to higher molecular weight and less reactive foams. Intermolecular bonding is also indicated for such resoles as freshly prepared for, despite relatively high initial viscosities which are sometimes observed, they are highly reactive as foamable compositions.
Irrespective of the cause of the increase in viscosity, lack of sufficient resole fluidity may either preclude processing to a foam or may hamper proper mixing of the components of the foam formulation. If adequate homogeneity of the resole with other ingredients such as the acidic catalyst, blowing agent and, when used, a surface active agent, is not realized, uneven foam rise occurs and the foam is of poorer overall quality than might otherwise be obtainable.
Among the blowing agents reported in the literature for phenolic foam formation are the polyhalogenated saturated fluorocarbons having a boiling point within the range from about minus 40.degree. F. up to about 200.degree. F. described in U.S. Pat. No. 3,389,094. Illustrative of this class are trichloromonofluoromethane, 1,1,2-trichloro-1,2,2-trifluoroethane (CCl.sub.2 FCClF.sub. 2), dichlorodifluoromethane, 1,2-difluoroethane and 1,1,1,2-tetrachloro-2,2-difluoroethane. Other halogenated blowing agents are chlorohydrocarbons such as methylene chloride and 1,2-dichloroethane. Another class of suitable foaming agents are the aliphatic ethers having a boiling point between 30.degree. C. and 100.degree. C. such as diethyl ether, diisopropyl ether and other such compounds described in U.S. Pat. No. 2,845,396. Of these conventional blowing agents, the chlorofluorocarbons such as in particular, trichloromonofluoromethane and 1,1,2-trichloro-1,2,2-trifluoroethane, have a tendency to give premixes with phenolic resoles which have viscosities greater than the resole itself. This may further complicate processability when the resole itself is relatively high in viscosity. Further, blowing agents commonly used in forming phenolic foams are usually insoluble in the resole raw material. It is believed that these incompatible premixes are, at least in part, controlling factors for formation of rigid phenolic foams which have a very fine cell structure relative to polyurethane foam. Such incompatibility is a disadvantage when it is desired to provide phenolic foam which is of larger cell size relative to fine cell phenolic foam but which is still of finer cell size relative to polyurethane foam.
It is an object of this invention, therefore, to provide an adjuvant for phenolic foam formation which allows for improvement in the processability of phenolic resoles, particularly resoles which lack sufficient fluidity for proper foaming.
Another object is to provide a processing aid for phenolic foam formation which has the aforesaid advantage and is additionally capable of functioning as a source of blowing action for the foaming operation.
Another object is to provide a blowing agent for phenolic foam which is compatible with phenol-formaldehyde resoles and which, despite its compatibility, allows for the formation of phenolic foam which has a finer cell structure than polyurethane foam.
A further object is to provide an improved method for the formation of cellular phenolic products.
Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.